182 SUMMARY OF CUKRENT RESEARCHES RELATING TO 



groups in the young' during constant body-weight. This is explained 

 as due to the presence of both the growth tendency and the (more or 

 less different) maintenance tendency in the young animals, whereas in 

 the adult there is only the tendency to maintenance. Both the growth 

 tendency and the maintenance tendency, however, show characteristic 

 differences in the various orscans according to nutritional conditions. 



Chorda tympani and Middle Ear in Reptiles, Birds and Mam- 

 mals.* — E. S. Goodrich discusses the chorda tympani and middle 

 ear in Reptiles, Birds and Mammals, with special reference to the exact 

 relation of the chorda tympani to the first gill-slit, tympanum and 

 surrounding structures. A comparison of the development of the 

 various structures of the middle ear region in the lizard, duck and 

 mammal shows a remarkable uniformity in their origin and relation. 

 The first gill-pouch separates off from the epidermis from below upwards ; 

 at its dorsal edge is an epiblastic proliferation contributing to the geni- 

 culate ganglion. The tympanum is formed between the outer epidermis 

 and an outgrowing diverticulum of the hinder lower region of the first 

 gill-pouch. The chorda tympani is a post-trematic branch of the facial 

 nerve, developing behind the first or spiracular gill-slit, and passing 

 down to the lower jaw between the tympanum and the closing spiracle. 

 The relation of these parts to the skeleton and blood-vessels is, except 

 in gaUinaceous birds, constant throughout the Amniota, and is only 

 intelligible on the view of Reichert that the proximal region of the 

 columella correspouds to the stapes, the quadrate to the incus, and the 

 articular to the malleus. In the duck the chorda tympani develops as 

 a pre-trematic branch of the facial nerve from its first appearance. In 

 adult gallinaceous birds the chorda passes down directly from the geni- 

 culate ganglion in front of the tympanic cavity. This exceptional 

 position is probably due to some secondary modification at present 

 unexplained. 



b, Histolo(?y. 



Roles of Nucleus and Cytoplasm in Melanin Elaboration.f — 

 Davenport Hooker discusses this question in the light of recent 

 research. He presents an account of the relevant investigations. It 

 appears that melanin may be produced from tyrosin, or its derivatives, 

 acted upon by an oxidizing agent, tyrosinase. Free tyi'osin occurs in 

 horses with melanotic tumours, and derivatives of tyrosin are absorbed 

 by the animal body. Lillie has shown that the nucleus of the cell 

 plays a part as a producer of oxygen or of an oxidase. Tyrosinase is 

 normally present in many pares of the body. In the case of the frog, 

 specially investigated by the author, it seems probable that the base 

 from which the melanin granules are formed exists in a soluble 

 condition in the cytoplasm, that the role of the cytoplasm is as a carrier 

 of the chromogen, and that the nucleus plays an all-important part. 



The evidence goes to show : — (1) that the theory of the origin of 

 melanin from chromatin extruded from the nucleus into the cytoplasm 



• Quart. Journ. Micr. Sci., Ix. (1915) pp. 133-60 (3 pis. and 5 figs.), 

 t Auat. Record, ix. (1915) pp. 393-402 (1 fig.). 



